Semiconductors having elements from Group II and Group VI of the periodic table, such as mercury cadmium telluride, have been used in the fabrication of infrared radiation detectors and imagers which operate in the lower infrared frequency band down to the limits of the available long wave length atmospheric window, i.e., at wavelengths of 8-12 microns. The detection of such long wavelength radiation, if it is to be done using a detector at only moderate cryogenic temperatures, e.g. at liquid nitrogen rather than liquid helium temperatures, is preferably done using a very narrow bandgap semiconductor, such as Hg.sub.1-x Cd.sub.x Te.
Since HgTe is a semimetal having a very small bandgap of about -0.3 eV, and CdTe has a bandgap of about +1.6 eV, compositions having a small and arbitrarily selectable bandgap may be specified simply by varying the proportions of an alloy having the composition Hg.sub.1-xCd.sub.x Te. Such alloys are here referred to generically as "HgCdTe". For example, for x=0.22 the composition Hg.sub.0.78 Cd.sub.0.22 Te has a cutoff of about 10 microns, that is, a composition having a bandgap approximately equal to the photon energy of infrared light having a wavelength of 10 microns. By increasing the percentage of cadmium, compositions having a larger bandgap, and therefore a shorter operating wavelength, may be produced. For example, the selection of x=0.3 yields the composition Hg.sub.0.7 Cd.sub.0.3 Te having a bandgap of about 0.24 eV which corresponds to a photon wavelength of 5 microns (at 77.degree. K.).